linux-kernel - Re: [patch 6/6] mm: memcg: print statistics from live counters

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Date:	Wed, 16 May 2012 16:01:31 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Johannes Weiner <hannes@...xchg.org>
Cc:	Michal Hocko <mhocko@...e.cz>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	cgroups@...r.kernel.org, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [patch 6/6] mm: memcg: print statistics from live counters

On Mon, 14 May 2012 20:00:51 +0200
Johannes Weiner <hannes@...xchg.org> wrote:

> Directly print statistics and event counters instead of going through
> an intermediate accumulation stage into a separate array, which used
> to require defining statistic items in more than one place.
> 
> ...
>
> -static const char *memcg_stat_strings[NR_MCS_STAT] = {
> -	"cache",
> -	"rss",
> -	"mapped_file",

Bah humbug, who went and called this mapped_file?

This stat is derived from MEM_CGROUP_STAT_FILE_MAPPED.  But if we
rename MEM_CGROUP_STAT_FILE_MAPPED to MEM_CGROUP_STAT_MAPPED_FILE then
we also need to rename the non-memcg NR_FILE_MAPPED.  And we can't
change the text to "file_mapped" because it's ABI.

> -	"mlock",
> -	"swap",

And "swap" is derived from MEM_CGROUP_STAT_SWAPOUT.  We could rename
that to MEM_CGROUP_STAT_SWAP without trouble.

But both are poor names.  There are two concepts here: a) swapout
events (ie: swap writeout initiation) and b) swapspace usage.  Type a)
only ever counts up, whereas type b) counts up and down.

MEM_CGROUP_STAT_SWAPOUT is actually of type b), but "swapout" is a
misleading term, because it refers to type a) events.

And the human-displayed "swap" is useless because it can refer to
either type a) or type b) events.  These should be called "swapped" and
MEM_CGROUP_STAT_SWAPPED.  But we can't change the userspace interface.

argh, I hate you all!

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